An expanding shell of ionized gas ejected from a red giant star in the late stages of stellar evolution. Despite the name, it has no connection to planets. The glowing gas is illuminated by the hot core of the dying star.
Source: science.nasa.gov

26/08/2012

28/06/2012

20/04/2012

03/04/2012

31/01/2012

14/01/2012

27/12/2011

13/11/2011

01/09/2011

16/08/2011

07/08/2011

28/07/2011

26/06/2011

24/04/2011

07/04/2011

18/02/2011

08/01/2011

03/11/2010

21/10/2010

14/09/2010

26/08/2010

23/07/2010

14/06/2010

12/06/2010

09/05/2010

06/05/2010

25/04/2010

11/04/2010

21/02/2010

31/12/2009